/*
 * Copyright (C) 2016 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import java.util.Comparator;
import java.util.Iterator;
import java.util.Optional;

/**
 * Provides static methods for working with {@link Comparator} instances. For many other helpful
 * comparator utilities, see either {@code Comparator} itself (for Java 8 or later), or
 * {@code com.google.common.collect.Ordering} (otherwise).
 *
 * <h3>Relationship to {@code Ordering}</h3>
 *
 * <p>
 * In light of the significant enhancements to {@code Comparator} in Java 8, the overwhelming
 * majority of usages of {@code Ordering} can be written using only built-in JDK APIs. Because of
 * this, and because it's awkward to have to convert comparators into {@code Ordering} instances,
 * {@code Ordering} and its methods are planned for deletion. This class is intended to "fill the
 * gap" and provide those features of {@code Ordering} not already provided by the JDK.
 *
 * @since 21.0
 * @author Louis Wasserman
 */
@Beta
@GwtCompatible
public final class Comparators {
    private Comparators() {}

    /**
     * Returns a new comparator which sorts iterables by comparing corresponding elements pairwise
     * until a nonzero result is found; imposes "dictionary order." If the end of one iterable is
     * reached, but not the other, the shorter iterable is considered to be less than the longer
     * one. For example, a lexicographical natural ordering over integers considers {@code
     * [] < [1] < [1, 1] < [1, 2] < [2]}.
     *
     * <p>
     * Note that {@code Collections.reverseOrder(lexicographical(comparator))} is not equivalent to
     * {@code lexicographical(Collections.reverseOrder(comparator))} (consider how each would order
     * {@code [1]} and {@code [1, 1]}).
     */
    // Note: 90% of the time we don't add type parameters or wildcards that serve only to "tweak"
    // the
    // desired return type. However, *nested* generics introduce a special class of problems that we
    // think tip it over into being worthwhile.
    public static <T, S extends T> Comparator<Iterable<S>> lexicographical(Comparator<T> comparator) {
        return new LexicographicalOrdering<S>(checkNotNull(comparator));
    }

    /**
     * Returns {@code true} if each element in {@code iterable} after the first is greater than or
     * equal to the element that preceded it, according to the specified comparator. Note that this
     * is always true when the iterable has fewer than two elements.
     */
    public static <T> boolean isInOrder(Iterable<? extends T> iterable, Comparator<T> comparator) {
        checkNotNull(comparator);
        Iterator<? extends T> it = iterable.iterator();
        if (it.hasNext()) {
            T prev = it.next();
            while (it.hasNext()) {
                T next = it.next();
                if (comparator.compare(prev, next) > 0) {
                    return false;
                }
                prev = next;
            }
        }
        return true;
    }

    /**
     * Returns {@code true} if each element in {@code iterable} after the first is <i>strictly</i>
     * greater than the element that preceded it, according to the specified comparator. Note that
     * this is always true when the iterable has fewer than two elements.
     */
    public static <T> boolean isInStrictOrder(Iterable<? extends T> iterable, Comparator<T> comparator) {
        checkNotNull(comparator);
        Iterator<? extends T> it = iterable.iterator();
        if (it.hasNext()) {
            T prev = it.next();
            while (it.hasNext()) {
                T next = it.next();
                if (comparator.compare(prev, next) >= 0) {
                    return false;
                }
                prev = next;
            }
        }
        return true;
    }

    /**
     * Returns a comparator of {@link Optional} values which treats {@link Optional#empty} as less
     * than all other values, and orders the rest using {@code valueComparator} on the contained
     * value.
     *
     * @since 22.0
     */
    @Beta
    public static <T> Comparator<Optional<T>> emptiesFirst(Comparator<T> valueComparator) {
        checkNotNull(valueComparator);
        return Comparator.comparing(o -> o.orElse(null), Comparator.nullsFirst(valueComparator));
    }

    /**
     * Returns a comparator of {@link Optional} values which treats {@link Optional#empty} as
     * greater than all other values, and orders the rest using {@code valueComparator} on the
     * contained value.
     *
     * @since 22.0
     */
    @Beta
    public static <T> Comparator<Optional<T>> emptiesLast(Comparator<T> valueComparator) {
        checkNotNull(valueComparator);
        return Comparator.comparing(o -> o.orElse(null), Comparator.nullsLast(valueComparator));
    }
}
